In a gas turbine plant, compressed air from a compressor, which is provided coaxially with a gas turbine, is introduced into a combustor together with fuel. High temperature combustion gas, which is caused by combustion of the fuel with the compressed air in the combustor, is introduced to turbine blades via transition pieces and turbine nozzles. The combustion gas drives turbine blades of the gas turbine to produce work to drive a generator, which is coupled to the gas turbine.
A heat resistant superalloy is applied to one or more components of the gas turbine, such as a combustion liner, a transition piece, a turbine blade or a turbine nozzle, which are exposed to the high temperature in the gas turbine. A nickel base superalloy is used for the turbine blade, where high temperature strength is especially needed. The nickel base superalloy is a precipitation strengthening type alloy, and its high temperature strength is achieved by precipitation of intermetallic compound of Ni3(Ai, Ti), which is referred to as “γ′ phase”, in a nickel matrix.
Although the nickel base superalloy has high heat resistance, however, various damages or defects (referred to as “damages”) may be observed in the nickel base superalloy after the gas turbine has been operated for certain period. This damage is caused by degradation of the material, erosion, corrosion or oxidization, which is more likely to occur for the gas turbine component such as turbine blade in the high temperature environment, in which the gas turbine components are exposed. Further, creep damages accumulate in the turbine blade due to centrifugal stress caused by the operation of the gas turbine. When the gas turbine plant starts or stops operating, further damages accumulate in the gas turbine blade by the thermal fatigues due to the change of temperature in addition to the centrifugal stress.
In general, a turbine blade is scrapped when it reaches its design life. For example, a turbine blade for the first stage of the 1,100 degrees centigrade class gas turbine operated for the base-load purpose having an oxidation-resistant and corrosion-resistant coating on its surface has 48,000 hours until it is scrapped. If the turbine blade is re-coated, such re-coating of the oxidation-resistant and corrosion resistant coating is carried out after 24,000 hours operation of the turbine blade. In that case, the re-coated turbine blade is used for 48,000 hours after the re-coating and is then scrapped. At the time of the re-coating, the turbine blade is heat-treated; however, this heat treatment is not intended to refurbish the base metal of the turbine blade.
Other turbine components exposed to the high temperature, such as the turbine nozzle, the combustion liner or the transition piece, are repaired by welding when a crack or an abrasion is found. These turbine components are used again after being repaired. When being repaired, these components are heat-treated to reduce the heat effect caused by welding or residual stress, if it is necessary.
Currently, the temperature of combustion gas introduced to gas turbine is becoming higher to improve the thermal efficiency. Hence the nickel base superalloy, which is used for turbine blade, is starting to be applied to the turbine nozzle, the combustion liner or the transition piece. It is generally known that the nickel base superalloy is difficult to repair or refurbish.
A conventional refurbishment technology for restoration of casting defects of the precision casting is described in the Japanese Patent Publication (Kokai) No. 57-207163. This technology is substantially a HIP process, which compresses defects such as creep voidsand dislocations. This publication describes a technique to perform heat treatment in a wide and general range of temperature (more than a temperature from 600 to 950 degrees centigrade) such as in a range of more than 50%, in a range of 60 to 95%, and in a range of 80-95% of melting point of the component (which is more than 1,000 degrees centigrade).
Japanese Patent Publication (Kokai) No. 51-151253 discloses a technology to remove small defects referred to as “creep voids” caused by creep in a metal component which has been used in the high temperature environment. In this technology, heat treatment is performed in a wide range of temperature, such as, 980 to 1232 degrees centigrade. This range covers the temperature from less than the solvus temperature of the γ′ phase to the highest temperature of the beginning of the incipient melting, and is not related with recovering of the γ′ phase.
Japanese Patent Publication (Kokai) No. 57-62884 discloses a technology that removes micro defects which are included inside the weld by HIP process after welding. However, heat treatment temperature range, which is disclosed as from about 1,000 to 1250 degrees centigrade, is also wide to cover the temperature from less than the solvus temperature of the γ′ phase to the highest temperature of the beginning of the incipient melting. This temperature range is not related with recovering of the γ′ phase.
Japanese Patent Publication (Kokai) No. 51-14131 discloses a uniformizing technology that eliminates an opening such as relatively large crack, a crevice or a large hole included in a casting just after its casting process.
Japanese Patent Publication (Kokai) No. 55-113833 discloses a technology applying solution heat treatment in addition to the HIP process to compress voids inside the casting.
However these two publications do not disclose a technology that enables to recover the microstructure of the alloy to the extent that is equivalent to the time of its manufacture, which has microstructure with full solution of the γ′ phases without defects caused by incipient melting, by applying solution heat treatment and aging heat treatment after the process of recovery heat treatment under high pressure to the gas turbine component used for the long period in the high temperature environment.
With regard to the gas turbine component of precipitation strengthening type alloy used in the high temperature environment, original property of the material such as creep life, ductility or toughness is reduced when the precipitation strengthening phases disappear or new precipitated phases precipitate by the process of changing shape of precipitated phases due to agglomeration, precipitation or enlargement of precipitated phases. In addition, creeps due to centrifugal or thermal stresses, thermal fatigues due to temperature-strain trajectory by startup/shutdown or the high/low cycle fatigues damage these gas turbine components.
Japanese Patent (Kokoku) No. 4-6789 and Japanese Patent Publication (Kokai) No. 2000-80455 disclose technologies of recovering life using heat treatment that recovers the microstructure of the alloy by elevating the temperature to the dissolving temperature of the γ′ phase, which constitutes coarsened main strengthening phase. However, because the incipient melting temperature and the solvus temperature of the γ′ phase are close to each other, the strength is reduced due to the incipient melting or the recrystallization. Further, the internal damage (defect) such as the creep void due to the operation cannot be eliminated with this process.
Regarding the material applied to the gas turbine component, an element that cause decline of the melting point tends to segregate along the dendrite boundary. Particularly, around the area where these elements extremely segregate along the dendrite boundary, the melting point is also extremely declined. In this case, the melting point of the area becomes close to the solving temperature of the γ′ phases, which are main precipitation strengthening phases. Therefore, these materials are usually heat-treated in the range of temperature that can make appropriate microstructure without causing incipient melting. Hence, it is difficult to recover the microstructure by re-precipitation of the γ′ phases, which are main precipitation strengthening phases, after its full solution. Rather, in some situations, it reduces strength or life of the component by further agglomerating γ′ phases that have already coarsened by use of the component.
Japanese Patent Publication (Kokai) No. 11-335802 discloses recovering technology that applies a recovery heat treatment process under high pressure environment using HIP process to restore inner defects and recover areas that have incipient melting before applying the solution heat treatment and the aging heat treatment, which are applied in a non-pressurized environment. In this technology, grain boundary strengthening elements such as B, Zr, Hf or C are added to the alloy. These elements segregate along the dendrite boundary during solidification process of alloy. This makes the temperature of incipient melting almost equivalent or less than the temperature of the solvus temperature of the γ′ phases. When the recovery heat treatment is applied to this alloy in the temperature higher than the solvus temperature of the γ′ phases, the γ′ phases can be fullly dissolved in the base metal, which is the γ phases. Since the recovery heat treatment process is applied in the high pressure environment, the pressure closes incipient melting areas even if the incipient meltings occur. Therefore, this technology enables to recover the alloy of the gas turbine component without reduction of the strength due to the incipient melting. The refurbished gas turbine components according to this technology can obtain life and property equivalent, or even greater than, compared to the time of its manufacture. However, in this technology, because the incipient melting area crystallizes and becomes fine grains during its solidification process, the recovering process may not be completed. Thus, recovery of the alloy according to this technology depends on the extent of the incipient melting.